The present invention relates generally to a digital impedance measurement system and, in particular, to a digital eddy current system used to facilitate measuring inductive ratios.
At least some known rotatable machines are monitored for a vibration using sensors that measure a change in displacement from a fixed position to a target fixed to a rotating member and/or from the rotating member itself. The sensors may be excited by conditioning circuitry to develop an output that is relative to a displacement and/or vibratory response of the target. The conditioning circuitry may be located remotely from the machine and sensor, and coupled to the sensor through one or more cables coupled together in series. At least some known systems include eddy current transducers to monitor proximity of a target to the transducer. The transducer is excited by the conditioning circuitry to interact with the target and such interaction causes a change in the electrical impedance of the transducer. The conditioning circuitry senses the change in electrical impedance and converts the change into a respective displacement and/or vibration output. The electrical impedance measured at the conditioning circuitry is the electrical combination of the target, the transducer including an integral sensing coil and cable, an extension cable and the conditioning circuitry. This impedance is usually called the “Tank Impedance” or parallel impedance (Zp).
Because the indicated gap between the transducer and the target is based upon the parallel impedance as sensed by the conditioning circuitry, any change that modifies the impedance of the transducer and/or cable as sensed by the conditioning circuitry may cause a false gap and/or vibration output from the system. For example, a change in the impedance of the transducer or of the cable connecting the transducer to the conditioning circuitry, may be due to factors not related to the gap, such as, wetting of the cable or transducer by water or another fluid. A fluid leaking into the cable or transducer may cause a change in the resistance and/or reactance of the transducer and/or cable. It would be advantageous to replace such cables and/or transducers. However, it may be impracticable due to machine operational commitments or the cable being located in a relatively extreme environment.